Gas turbines are used in a variety of industrial settings to supply power to a load, generally an electrical generator. A gas turbine consists of a compressor to compress air and an expander to recover energy from the compressed air after having been heated. The compressed air is heated within a set of combustors located between the compressor and the expander by supporting combustion of a fuel introduced into the combustors.
There are a variety of prior art examples in which the potential power output of a gas turbine is increased by water injection into the compressor, the combustor or both. Water injection into the compressor increases the efficiency of the compressor and thus reduces the amount of energy drawn from the expander to compress the air. Water injection into the combustor increases the amount of mass being expanded and therefore, the power output of the gas turbine.
For example, U.S. Pat. No. 4,667,467 utilizes water injection to the compressor to provide high efficiency electric power production in connection with the gasification of coal. In this patent, water spray intercooling of the gas turbine compressor is used along with an evaporative spray cooler to add water to the air to be compressed in the compressor. The resultant compressed, air-water mixture is used to a minor extent in the coal gasifier. The major part of such mixture is fed into a combustor. While undoubtedly, high gas turbine efficiencies are realized, the amount of potential increase of the power output is limited due to the fact that the compressor is coupled to the expander and for given atmospheric conditions there is only so much water that can be fed to the compressor. Water liquid, as opposed to water vapor could destroy the gas turbine due to blade erosion. Hence, since the amount of water fed to the compressor is limited, the amount of water potentially fed to the combustor and the expander is also limited.
U.S. Pat. No. 6,038,849 discloses a system for augmenting the power produced by a gas turbine by introduction of water into the combustor along with supplemental compressed air produced by a separate compressor train. The system disclosed in this patent is specifically designed to increase the power output of a gas turbine due to power losses occurring at high ambient temperature and elevation or in other words, conditions of low air density. In one embodiment, the separate compressor train utilizes interstage cooling to increase the efficiency of the compressor train. The compressed air, which can be further cooled, is then introduced into a saturator and then preheated. The preheated air, thereby containing water vapor, is introduced into the gas turbine combustor. The efficiency of power augmentation that is possible is limited in this patent in that although interstage cooling of the separate compressor train reduces power input, an energy loss exists at the intercooler which rejects heat between the compression stages to the ambient. In fact, the patent contemplates yet further cooling of the compressed air in an aftercooler that would further reject the heat of compression.
As will be discussed, the present invention provides a power augmentation system for a gas turbine in which water injection is utilized in a manner that overcomes such problems of the prior art as have been discussed above.